The present invention relates to computer display systems, and particularly to methods and systems for displaying interlaced video on monitors which are non-interlaced.
Until now there have been two commonly used simple methods for displaying interlaced video being fed into the computer system on a computer monitor. These are normally independent of whether the computer monitor is interlaced or not, as even when the monitor is interlaced it normally refreshes at a rate independent of the incoming video signal.
Throughout this description NTSC video is assumed for the sake of illustrative examples, with references to 240 line fields, 480 line frames, 60 fields per second and 30 frames per second. This does not restrict the invention to NTSC or the line counts or frame or field rates but is merely used for simplicity. The invention is equally applicable to other video standards such as, but not limited to, PAL with 288 line fields, 576 line frames, 50 fields per second and 25 frames per second.
The first method is just capturing one of the two fields, and displaying 240 lines scaled (interpolated) up to 480 or however many are in the current display mode. The special case of scaling to 480 lines (line doubling) is currently used in the art and is well documented. See pages 332-333 of xe2x80x9cVideo Demystified: a Handbook for the Digital Engineersxe2x80x9d by Keith Jack, HighText Publication Inc., 1993 (referred to herein as xe2x80x9cKeith Jackxe2x80x9d).
The second method is to perform simple de-interlacing where both fields are captured into a single 480 line buffer and double the buffer line length for a single field in order to store a field in every other line. This is referred to as xe2x80x9cField Mergingxe2x80x9d (see p. 333 of Keith Jack)
(1) Deinterlacing by interleaving two fields into a single buffer
This method is fine in theory and provides better vertical resolution than a single field 240 line capture, but gives very objectionable results when viewing video with rapid horizontal action (for example a football game), as noted in page 333 of Keith Jack.
As the two fields of a single video image are separated in time by {fraction (1/60)}th of a second, storing the later field interweaved into the same buffer results in a image with a zipper like appearance along high contrast vertical edges when rapid horizontal motion takes place in the source video. This effect is illustrated in FIG. 1 of the present application. Page 335, FIG. 7 of Keith Jack uses a picture of a flying bird to illustrate this artefact.
Another problem to compound these line to line zipper like artefacts occurs when one needs to scale the resulting 480 line video up to different sizes. When one scales up by line replication, at certain points in the vertical scaling it is necessary to display one of the lines twice. If the image already has a repetitive left-right-left-right-left-right offset on vertical edges, replicating a single line introduces what appears as another break in the video. The effect can be seen in FIG. 2.
These breaks appear in a regular pattern, dictated by the scaling factor used to scale up from 480 lines to the destination size (for example 600 or 768 lines).
Pages 333-336 of Keith Jack refer to advanced techniques requiring detection of motion between fields on a pixel by pixel basis requiring the storage of 4 fields. This processing on a pixel by pixel basis would typically be expensive to implement because of the requirement for storing 4 fields and attempting to compare and process pixels from two fields to generate each output pixel.
Vertical interpolation can help to reduce this second artefact by attempting to interpolate between the lines from the two fields, but still does not give visually pleasing results.
(2) Displaying a single field per frame
Displaying a single field from a frame has an advantage, but some definite problems. The advantage is that there are none of the artefacts described above relating to the interleaving of two time-separated fields into a single buffer. There are three main problems. The first problem is that the image generated is fundamentally lower resolution vertically, coming from only 240 lines. Keith Jack refers to this when discussing xe2x80x9cScan Line Duplicationxe2x80x9d and xe2x80x9cScan Line Interpolation,xe2x80x9d indicating that although the number of lines is doubled, the vertical resolution is not increased from the original data (see pages 332-333 of Keith Jack). In addition, Keith Jack only deals with displaying on a 480 line display where the number of lines is exactly doubled. Further, Keith Jack only considers displaying a single field because it does not consider the differing spatial aspects of odd and even fields in an interlaced video source.
The second problem is that the image only changes 30 times per second, whereas the source interlaced data changes 60 times per second. Thirty frames per second is often considered xe2x80x9cfull motion video,xe2x80x9d indicating that it is good enough to fool the human eye into perceiving smooth motion. However, performing a side by side comparison of 30 and 60 frame per second video makes it apparent that 60 frames per second is noticeably smoother.
The third problem is that displaying a single field at a rate of 30 fields per second interferes with 3:2 pull down commonly used to transmit films shot at 24 frames per second on an NTSC signal at 60 fields per second. With 3:2 pull down, a single film frame is transmitted for either two or three NTSC fields in order to approximate to the nominal 30 frames per second of NTSC. FIG. 3 shows the relationship of the film, the transmitted NTSC fields, and the images displayed on the computer screen.
It can be seen from column C of FIG. 3 that on a conventional TV the successive frames are displayed for 50 ms, 33.3 ms, 50 ms, 33.3 ms, 50 ms, and so on. This rapid alternation between two display times which differ by a factor of 1.5 gives a good impression of smooth motion on a conventional TV.
From column E of FIG. 3, it can be seen that using and displaying a single field results in the successive frames being displayed for 66.6, 33.3, 33.3, 33.3, 66.6 ms, and so on. Note that the period of the alternation between the short display time and the long display time is two times that of column C, and that the two display times differ by a factor of 2.
Summarizing, column E has a higher variability in the display time and a longer period in the variability. These two factors result in a noticeable jerkiness in the displayed images, particularly in smooth but rapid horizontal pans in the source film.
Note that in this simplistic analysis the effect of running the video monitor attached to the computer at a rate other than 60 Hz has been ignored. If the monitor is being refreshed at another frequency, (for example the commonly used 75 Hz), the artefacts introduced vary somewhat but the displayed images still show a fundamental jerkiness.
Pages 358-361 of Keith Jack address issues regarding field and frame rate conversion, but only for conversion from a computer monitor non-interlaced to TV interlaced, or from one interlaced standard to another. Keith Jack does not address frame rate conversion from interlaced (e.g., TV) to non-interlaced (e.g., computer monitor) systems. In addition, it refers to the xe2x80x9c3:2 pull downxe2x80x9d technique illustrated above in columns A and B (see its section on Field and Frame Rate Conversion of page 361 and FIG. 9.36 of page 365) for conversion from 24 frames per second film to 60 fields per second NTSC.
The following inventions describe ways of displaying interlaced video from sources such as MPEG 1, MPEG 2, Broadcast TV, Cable TV, Satellite TV, Direct Broadcast Satellite (PBS), Direct Satellite System (DSS), Video Tape Recorders (VTR""s ), LaserDisc, and any other sources of interlaced video, along with non-interlaced MPEG 1 video, on computer systems.
One method used in the present invention is to display all of the incoming fields but one at a time, and correcting for the positional offset of one field relative to another in the interlaced data. The method of doing this is to capture the two fields into separate buffers, one for the odd field and one for the even field. When one of the fields has been captured into the buffer, the buffer is displayed, scaled to the requested dimensions on the monitor using some scaling hardware or software. That image is displayed until the next field is captured into another buffer, and then the subsequent image is displayed until the third field is captured into either the original first buffer, or into another (third) buffer.
An important aspect of the present invention is the correction of the positional offset of the two interlaced video fields. There are two ways presented to deal with the vertical offset of the two fields in accordance with the present invention. The first way is that the two fields can be displayed at different positions on the display using a non-interlaced display. The second way is that the video data can be altered to correct the positional offset between the fields.
Another method of the present invention is to lock the frame rate of the output video to the incoming field rate or a multiple of the incoming field rate, or to certain sub-multiples of the incoming field rate. This is a much looser coupling of rates than conventional genlocking, and consequently can be implemented much more cheaply. All that is required to ensure that each field is displayed for the predetermined number of frame times on the output monitor. If the output frame rate is being made the same as the incoming field rate, then each field needs to be shown exactly once. This results in a frame rate of the output display of exactly the incoming field rate (59.94 hertz for NTSC, 50.00 hertz for PAL and SECAM). Similarly, for an output monitor rate of twice the incoming field rate, each field is displayed for exactly two output frames.
An important feature of this method is that each frame of the output monitor need not match the incoming field time precisely. As long as each output frame is displayed exactly the predetermined number of times, the appearance of smooth motion will be maintained.
These and other features of the present invention will become apparent from the following description when read in conjunction with the drawings and the appended claims.